fix(sl): parse user filter expressions as predicates, not projections (#307)

* fix(sl): parse user filter expressions as predicates, not projections

User-authored filters and segments were parsed in a projection context
(`SELECT {expr}`). On T-SQL a top-level `col = 'value'` projection is the
`alias = expression` aliasing syntax, so an equality filter parsed this way
became `'value' AS col` — dropping the comparison entirely and silently
skipping computed-column expansion (the column hid behind the alias).

Parse user fragments as predicates (`SELECT * WHERE {expr}`) at every parse
site — the parser cache, measure-filter CASE WHEN generation, computed-column
expansion, and measure-filter/segment column qualification. For plain
non-condition expressions the column set is identical, so this is a no-op
everywhere except the T-SQL alias case it fixes.

Add cross-dialect regression tests (tsql, postgres, snowflake, bigquery)
locking equality filters/segments to comparison shape and confirming `= 'x'`
now matches `IN ('x')` on T-SQL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Shorten T-SQL predicate comments

* docs(sl): tighten T-SQL predicate docstrings and AGENTS docstring rule

Trim the parser and regression-test docstrings to the 1-3 line bar and
extend the AGENTS.md comment guidance to cover docstrings explicitly.

* refactor(sl): route all filter parsing through parse_predicate

Consolidate the predicate-context parse into a single parse_predicate
helper and route every filter-parsing call site through it: measure
CASE-WHEN filters, segments, computed-column-in-filter, the
aggregate-locality HAVING rewrite, and the planner OR-mixing /
top-level-AND split. The locality and split paths still parsed user
filters in projection context, so a named-measure equality filter
compiled to `0 AS measure` on T-SQL. Add a locality regression test
covering the HAVING rewrite path.

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Andrey Avtomonov <andreybavt@gmail.com>

python/ktx-sl/semantic_layer/generator.py

@@ -15,7 +15,11 @@ from semantic_layer.models import (
     ResolvedPlan,
     SourceDefinition,
 )
-from semantic_layer.parser import ExpressionParser, quote_reserved_identifiers
+from semantic_layer.parser import (
+    ExpressionParser,
+    parse_predicate,
+    quote_reserved_identifiers,
+)
 
 # DIALECT CONVENTION:
 #   User-authored SQL fragments (measure `expr`, segment `expr`, filter,
@@ -673,9 +677,7 @@ class SqlGenerator:
             if isinstance(select_expr, exp.Alias):
                 select_expr = select_expr.this
 
-            filter_cond = sqlglot.parse_one(
-                f"SELECT {filter_sql}", read=self.dialect
-            ).expressions[0]
+            filter_cond = parse_predicate(filter_sql, self.dialect)
 
             def _make_case(inner_node):
                 return exp.Case(
@@ -1073,10 +1075,7 @@ class SqlGenerator:
 
         # AST-based rewriting for robustness
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT {quote_reserved_identifiers(filter_expr)}",
-                read=self.dialect,
-            )
+            condition = parse_predicate(filter_expr, self.dialect)
 
             def _rewrite(node):
                 if isinstance(node, (exp.AggFunc, exp.Anonymous)):
@@ -1099,8 +1098,8 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            transformed = tree.transform(_rewrite)
-            return transformed.expressions[0].sql(dialect=self.dialect)
+            transformed = condition.transform(_rewrite)
+            return transformed.sql(dialect=self.dialect)
         except Exception:
             logger.debug(
                 "AST-based HAVING rewrite failed for locality filter, falling back to regex: %s",
@@ -1292,9 +1291,7 @@ class SqlGenerator:
             return expr
 
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT {quote_reserved_identifiers(expr)}", read=self.dialect
-            )
+            condition = parse_predicate(expr, self.dialect)
 
             changed = False
 
@@ -1309,9 +1306,9 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            transformed = tree.transform(_replace)
+            transformed = condition.transform(_replace)
             if changed:
-                return transformed.expressions[0].sql(dialect=self.dialect)
+                return transformed.sql(dialect=self.dialect)
         except Exception:
             logger.debug("AST-based computed column expansion failed for: %s", expr)
 
@@ -1357,13 +1354,7 @@ class SqlGenerator:
 
         # Use AST to find and replace column references matching measure names
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(f)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return f
+            condition = parse_predicate(f, self.dialect)
 
             changed = False
 
@@ -1388,7 +1379,7 @@ class SqlGenerator:
                         ).expressions[0]
                 return node
 
-            new_where = where.this.transform(_replace)
+            new_where = condition.transform(_replace)
             if changed:
                 return new_where.sql(dialect=self.dialect)
         except Exception:

python/ktx-sl/semantic_layer/parser.py

@@ -196,6 +196,15 @@ def quote_reserved_identifiers(expr: str) -> str:
     return result
 
 
+def _predicate_select(expr: str) -> str:
+    """Wrap a user expression as `SELECT * WHERE …`, quoting reserved identifiers.
+
+    Predicate, not projection: T-SQL reads a top-level `col = 'value'` projection
+    as the `alias = expression` form and would compile the filter to `'value' AS col`.
+    """
+    return f"SELECT * WHERE {quote_reserved_identifiers(expr)}"
+
+
 @functools.lru_cache(maxsize=256)
 def _cached_parse_select(sql: str, dialect: str) -> exp.Expression:
     """Cache parsed SELECT wrapper trees keyed by (sql, dialect).
@@ -206,6 +215,14 @@ def _cached_parse_select(sql: str, dialect: str) -> exp.Expression:
     return sqlglot.parse_one(sql, read=dialect)
 
 
+def parse_predicate(expr: str, dialect: str) -> exp.Expression:
+    """Parse a user filter into a fresh, mutable WHERE-condition node.
+
+    Uncached, so the result is safe to `.transform()`; raises on unparseable input.
+    """
+    return sqlglot.parse_one(_predicate_select(expr), read=dialect).find(exp.Where).this
+
+
 class ExpressionParser:
     """Parses user-authored SQL expressions for AST walks.
 
@@ -218,12 +235,9 @@ class ExpressionParser:
     def __init__(self, dialect: str = "postgres") -> None:
         self.dialect = dialect
 
-    def _quote_reserved_identifiers(self, expr: str) -> str:
-        return quote_reserved_identifiers(expr)
-
-    def _parse_as_select(self, quoted_expr: str) -> exp.Expression:
-        """Parse expression wrapped in SELECT, using cache for repeated expressions."""
-        return _cached_parse_select(f"SELECT {quoted_expr}", self.dialect)
+    def _parse_as_select(self, expr: str) -> exp.Expression:
+        """Parse a user fragment for read-only AST walks, via the parse cache."""
+        return _cached_parse_select(_predicate_select(expr), self.dialect)
 
     def parse(
         self,
@@ -236,8 +250,7 @@ class ExpressionParser:
         if not expr or not expr.strip():
             return result
 
-        quoted_expr = self._quote_reserved_identifiers(expr)
-        tree = self._parse_as_select(quoted_expr)
+        tree = self._parse_as_select(expr)
 
         # Extract source.column references
         for col in tree.find_all(exp.Column):
@@ -296,8 +309,7 @@ class ExpressionParser:
         """Quick extraction of source names from an expression."""
         if not expr or not expr.strip():
             return set()
-        quoted_expr = self._quote_reserved_identifiers(expr)
-        tree = self._parse_as_select(quoted_expr)
+        tree = self._parse_as_select(expr)
         return {
             _strip_quotes(col.table) for col in tree.find_all(exp.Column) if col.table
         }

python/ktx-sl/semantic_layer/planner.py

@@ -22,7 +22,11 @@ from semantic_layer.models import (
     SemanticQuery,
     SourceDefinition,
 )
-from semantic_layer.parser import ExpressionParser, quote_reserved_identifiers
+from semantic_layer.parser import (
+    ExpressionParser,
+    parse_predicate,
+    quote_reserved_identifiers,
+)
 
 # DIALECT CONVENTION:
 #   User-authored measure `expr`, `filter`, and computed-column fragments must
@@ -910,9 +914,7 @@ class QueryPlanner:
         for c in source.columns:
             col_to_source[c.name] = source_name
 
-        tree = sqlglot.parse_one(
-            f"SELECT {quote_reserved_identifiers(expr)}", read=self.dialect
-        )
+        condition = parse_predicate(expr, self.dialect)
 
         def _qualify_column(node):
             if (
@@ -926,8 +928,8 @@ class QueryPlanner:
                 )
             return node
 
-        transformed = tree.transform(_qualify_column)
-        return transformed.expressions[0].sql(dialect=self.dialect)
+        transformed = condition.transform(_qualify_column)
+        return transformed.sql(dialect=self.dialect)
 
     def _detect_fan_out(
         self,
@@ -1254,14 +1256,7 @@ class QueryPlanner:
     ) -> None:
         """Raise an error if an OR expression mixes WHERE and HAVING conditions."""
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(clause)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return
-            inner = where.this
+            inner = parse_predicate(clause, self.dialect)
             # Only check if the top level contains OR
             or_parts: list[str] = []
 
@@ -1295,14 +1290,7 @@ class QueryPlanner:
     def _split_top_level_and(self, expr: str) -> list[str]:
         """Split a filter expression on top-level AND (not inside parentheses or strings)."""
         try:
-            tree = sqlglot.parse_one(
-                f"SELECT * WHERE {quote_reserved_identifiers(expr)}",
-                dialect=self.dialect,
-            )
-            where = tree.find(exp.Where)
-            if not where:
-                return [expr]
-            inner = where.this
+            inner = parse_predicate(expr, self.dialect)
             parts: list[str] = []
 
             def _collect_and(node):